engine vibration mystery

[Ivan]

I have a YS91 ST in my freya, and when I hit about 1750 headspeed, I get a terrible vibration, which makes the tail fin buzz terribly. I have a new tail hub and shaft, have the fan hub dialed in to .0015 or less, same with clutch shaft. I have checked and re checked everything. I am wondering if there is something I am missing that makes th engine run rough.

YS 91 ST
8.1 gearing (i think..)
OS 8 plug
Magnum Heli Plus fuel

new bearings in it before today. I even took the hex coupler off and made a one way bearing start shaft.

Any Ideas?

[archiebald]

If its only happening at a specific rpm it more likely resonance. i.e. the vibration from the engine that might be there all through the rev range just happens to coincide with the natural frequency of vibration of the tail at that rpm.

As an experiment you could try doing something with the mounting of the fin, or even try adding a small weight to the fin so that it shifts the natural frequency.

[Ivan]

I dont think its harmonics. The vibration is persistant in all rpm ranges, and gets worse as rpm increases. The reference to the tail fin buzz was just where it materializes. There are other symptoms, such as buzzing control rods, links popping loose in flight (really entertaining to deal with) and fuel foaming.

I might try a cooler plug and see if that makes a difference. The engine sounds rough as well, even when it is plenty rich, it crackles. might be detonation or something. It is acting quite strange.

[chrsvct]

I just read somewhere that if you increase the head shims (lower the compression) it will take some of the vibration away, get the head shims to measure .008-.012 I havnt tried it yet on mine